In various fields, such as aerospace, vehicles (automobiles, motor cycles, trains), medical instruments, welfare devices, and robots, low weights of parts are desired for improvement of function, improvement of performance, and improvement of operability. Specifically, in the field of vehicles such as automobiles, emission amounts of carbon dioxide has been desired to be reduced in concern for the environment. Therefore, the need to reduce weight to reduce fuel consumption has become increasingly stringent every year.
Development of lightweight parts has been active primarily in the field of vehicles, and great strengthening of steels by improvements in composition, surface modifications, and combination thereof in steels has been primarily researched. For example, high-tension steels have been primarily used for springs, which are typical strong parts, and fatigue strength thereof is further improved by applying surface modification such as nitriding and shot peening, thereby yielding lightweight springs. However, great strengthening of steels by conventional improvements in composition is nearing limits, and great reductions in weight in the future cannot be anticipated.
Therefore, lightweight alloys, typically having low specific gravity, such as titanium alloys, aluminum alloys, and magnesium alloys are desired for further reduction in weight. Magnesium alloys have the lowest specific gravity in the practical metals, which is about 1/4 of that of steels, about 1/2.5 of that of titanium alloys, and about 1/1.5 of that of aluminum alloys. Therefore, magnesium alloys have great advantages in being low in weight and as a resource, and they are expected to be widely used in the market.
However, conventional magnesium alloys are limited in use as products. The main reason for this is that the strengths of the conventional magnesium alloys are low. Therefore, in order to obtain strength for parts, it is necessary to increase size of parts compared to that of the conventional steel parts. That is, the conventional magnesium alloys have not been accepted as strong parts in the market since low weight and compact size are incompatible.
Under such circumstances, research in high-strength magnesium alloys for use as strong parts has been actively made. For example, Patent Document 1 discloses a technique in which a molten Mg—Al—Zn—Mn—Ca-RE (rare earth) alloy is subjected to wheel casting, thereby forming a solid member, which is drawn and densified, thereby obtaining a magnesium alloy member having a 0.2% proof stress of 565 MPa.
Patent Document 2 discloses a technique in which a molten Mg—X-Ln (X is one or more of Cu, Ni, Sn, and Zn; Ln is one or more of Y, La, Ce, Nd, Sm) alloy is rapidly cooled and solidified, thereby yielding an amorphous foil strip composed of a magnesium alloy foil strip having a hardness of 200 HV or more.
Patent Document 3 discloses a technique in which a cast material or an extruded material composed of a Mg—Al—Mn alloy is drawn, thereby obtaining a magnesium alloy wire having a tensile strength of 250 MPa or more and an elongation of 6% or more.
Patent Document 1 is Japanese Unexamined Patent Application Publication No. 3-90530. Patent Document 2 is Japanese Unexamined Patent Application Publication No. 3-10041. Patent Document 2 is Japanese Unexamined Patent Application Publication No. 2003-293069.
The techniques disclosed in the publications are effective for greatly strengthening magnesium alloys. However, in the magnesium alloy disclosed in Patent Document 1, mechanical properties for satisfying requirements of the market for strong parts are not sufficient. For example, when it is assumed that the alloy is used in a spring in which at least one of bending stress and twisting stress primarily acts, according to estimates by the inventors, the magnesium alloy wire rod must have a 0.2% proof stress of 550 MPa or more in an inner portion of the wire rod and a 0.2% proof stress of 650 MPa or more in the vicinity of the surface of the wire rod if the size of the wire rod is the same as that of existing steel springs and light weight can be achieved. Furthermore, in order to form a coiled spring, at least an elongation of 5% or more in an inner portion is required. However, in the alloy member disclosed in Patent Document 1, which has the highest 0.2% proof stress of 565 MPa, the ductility is low and the elongation is only 1.6%. On the other hand, in the alloy member disclosed in Patent Document 1, which has the highest ductility and an elongation of 4.7%, the elongation is close to the value that is required in the present invention. However, the strength is low in a 0.2% proof stress of 535 MPa, and the requirement is not satisfied.
In the magnesium alloy disclosed in Patent Document 2, a hardness of 170 HV or more is obtained. The hardness corresponds to 0.2% proof stress of 650 MPa or more according to estimates by the inventors. However, in Patent Document 2, properties related to ductility are not disclosed. The magnesium alloy disclosed in this publication contains a large amount of rare earth elements and 50% of amorphous phase, whereby the ductility is extremely low, and it is easily assumed that sufficient elongation is not obtained. Furthermore, amorphous phases show poor thermal stability and easily crystallize by external causes such as environmental temperature. Since a mixed-phase alloy of amorphous phase and crystal phase causes large variation in the properties according to the proportion of the phases, it is difficult to stably produce products having uniform properties, and it is not suitable for applying to industrial products because of difficulty in guaranteeing quality in the market.
In the magnesium alloy disclosed in Patent Document 3, the elongation is 6% or more and shows sufficient ductility. However, the tensile strength is 479 MPa at most, and the above-mentioned 0.2% proof stress of 550 MPa or more in the inner portion of the wire rod is not obtained.
In order to improve fatigue strength of parts in which at least one of bending stress and twisting stress primarily acts, it is effective to apply the compressive residual stress on the surface thereof by shot peening, etc. However, in the conventional magnesium alloys, it is difficult to obtain sufficient compressive residual stress to improve the fatigue strength, since yield stress (that is, 0.2% proof stress) thereof is low. For example, the applied compressive residual stress is about 50 MPa or less, even if the compressive residual stress is applied to a conventional magnesium alloy by shot peening.